The immunology , immunochemistry and cellular biology of two parasites that cause considerable disease in the United States are studied. Giardia lamblia is the most common disease-causing parasite in the United States and one of the earliest branching protist that possesses a unique secretory system. Giardia lamblia undergoes surface antigenic variation where only one of a family of structurally related proteins(variant-specific surface proteins, VSPs) is expressed on the surface at any one time. There are a number of conserved motifs in the family including a one or two GGCY motifs, a Zn finger motif and a well conserved hydrophobic transmembranous portion followed by an absolutely conserved terminal 5 amino acids, CRGKA . Anlaysis of VSPS suggest that the amino terminal portion of VSPs are most diverse and antigenically distinct compared to the relatively well conserved carboxyl terminal portion. However, why he hydrophobic membranous portion is so well conserved and why the terminal 5 amino acids are absolutely conserved is not known. The terminal aminoacids is not required for proper insertion of VSP into the surface nor is the precise hydrophobic transmembrane sequence. However, conservation suggest these regions have important biological functions. Two post translational modification were characterized. Previous studies by others showed that the VSPs are palmitolylated. Using a combination of transfection of VSP H7, HA and transfected into WB labeled with palmitate, we showed that VSPH7 was palmitolyated. Mutation studies revealed that palmitoylation occurred on the ?C? of CRGKA. WB Giardia expressing mutated non palmitoylated VSPH7 as well as its own native VSPs appeared unchanged. However, palmitoylation changed the membrane domain and partitioning of VSPs suggesting that this may be a one biologically function. Experimental are planned to expressed both palmitolyated and non palmitolyated in mammalian cells to determine if the palmitolyation permits partitioning into RAFTs. The palmitoyl transferase responsible for VSP palmitoylation was identified and characterized. This transferase was able to transfer palmitoylate to in vitro translated VSPH7 and knockdown of this enzyme in vivo decreased palmitolylation of in vitro translated VSPH7. The second post translational modification that was discovered and characterized is the coversion of arginine in the CRGKA of VSPs to citrulline by arginine deiminase. Arginine deiminase in almost all prokaryotes and also G functions in the coversion of free arginine to citrulline. Other studies in Giardia also showed it functioned similar to most bacterial deiminases. However, in mammals a peptidylarginine functions to convert protein bound arginine to citrullin and therefore is an enzyme is a post translation modification of VSPs. As mentioned above, the conserved nature of CRGKA VSP tail suggested a biological function and one way to determine the function was to use a poly-his CRGKA to pull down associated proteins. This technique yielded arginine deiminase. Two different VSPs tested both contained citrulline and as a more definitive proof purified recombinant Giardial arginine deiminase converted the arginine in the hist-CRGKA peptide to citrulline. Furthermore, Giardia arginine deiminase localized to the plasma membrane and colocalized with VSP as would be expected. Surprisingly, the enzyme localized to the nuclei during encystations. The biological function of the conversion to protein bound arginine to citrullin is not understood in most situations, but promises to be important in signaling and protein interactions.